Linkages between 200-mb Tropical and Extratropical Circulation Anomalies during the 1986–1989 ENSO Cycle

Abstract

NMC operational wind analyses have been used to document the 200-mb circulation anomalies during the solstice seasons of the 1987–1989 ENSO cycle. Using the one-level vorticity balance diagnostic approach, the 200-mb Rossby wave anomalous source distribution was derived as departures from a three-year mean for the single warm-phase season (January–March 1987). The effective Rossby wave anomaly sources were found to be generally quite small within 15° of the equator. Poleward of about 15°, the anomalous 200-mb divergence, through the planetary vorticity-divergence anomaly stretching term, is the dominant factor in determining the anomalous 200-mb vorticity source distribution. Its JFM standard deviation around 30°−35°N was an order of magnitude larger than that found in the deep tropics. The January–March and June–August 200-mb circulation differences between the warm and cold phase of the 1987–1989 ENSO cycle are described. The ENSO cycle linkages between the equatorial Pacific convection anomalies and the extratropical circulation are viewed in terms of regional Hadley component anomalies and tropical-extratropical divergence anomaly zones (TEDAZs), which typically extend eastward and poleward from the equatorial convection anomalies into the subtropics and extratropics. The larger Coriolis parameter at higher latitudes can result in significant vorticity anomaly sources in the subtropical branches of the anomalous Hadley circulations and in the extratropical extensions of the TEDAZS. The information obtained from this case study is synthesized into a coherent description of linkages between the eastern equatorial Pacific and the Northern Hemisphere circulation during the northern winter. The prime longitudinal belt of coupling extends from near the date line eastward to the central or eastern Atlantic. The primary ingredients of the coupling are 1) the subsiding branch of the anomalous Hadley circulation in the North Pacific, 2) enhanced downstream subtropical westerlies across the Gulf of Mexico into the Atlantic, 3) an equatorward enhancement of 200-mb synoptic-scale variability associated with the enhanced westerlies, 4) coupling between the anomalous eddy activity in the eastern subtropical Pacific and the anomalous equatorial convection. The resulting northeastward extension of the anomalous equatorial convection across Mexico into the Atlantic gives rise to a time-averaged anomalous divergent circulation whose upward branch coincides with the region of enhanced convection and whose primary subsiding branch extends from northeastern South America northeastward across the low-latitude North Atlantic. This linkage with the Northern Hemisphere subtropics contrasts with most previous descriptions, which associate the low-latitude South American-Atlantic anomalies more directly with the Pacific anomalies through an anomalous Walker circulation.